The increasing need for data communication services is being accommodated today by using both separate data networks and by integrating data communication services over existing voice communication (telephone) networks. Some of these separate data networks, however, still rely on existing voice facilities for local access to their network. This ever increasing penetration of data services into the local voice facilities creates network traffic problems for the existing telephone network. These problems occur because the telephone network was initially designed for voice communications which has traffic characteristics that are different than those of data communications.
Data calls differ from voice calls primarily in the holding time (off-hook time) of the call and the amount of information conveyed per unit time during the call. The holding time, which is the length of time a call lasts, on average is much longer with data calls than with voice calls which typically last three to four minutes. This mismatch in the holding time characteristic causes usage problems in the telephone network and service problems for the user.
Data services can be categorized into three basic types. One type communicates a single data transaction per data call. An example of such a system is the transaction telephone as described in the article entitled "Transaction Telephone Gets the Facts at the Point of Sale", Bell Laboratories Record, at page 377 et seq., October, 1975 by V. Scott Borison. Using the transaction telephone the dialing and customer identification information are entered from a magnetically encoded card. In the transaction system the user goes off-hook, enters the dialing card information, enters the customer's identification card information and manually enters the transaction data. The computer's verification response is either visual or audible after which the telephone is placed on-hook. The above arrangement requires only a short holding time for each data call and enables the same telephone facility to be shared between voice and data services.
A second type of data service involves multiple interactive transactions to one computer center. This type of service typically involves going off-hook, dialing the computer center, entering the customer's identification, and entering a log-in sequence. This is followed by several data entries and computer responses conducted over a period of time. With this type of data service the telephone facility remains off-hook during the time when no data is being communicated. It is desirable for the data service to keep the line off-hook to both assure accessability to the data service computer on demand and to minimize the resulting protocol overhead (computer dialing customer's identification, log-in and log-out) which occurs when each data transaction is made using a separate data call. However, with such an arrangement the holding times are very long for each data call. Additionally, the long holding times make it impractical to share voice and data service over the same facility. Typical of this type of data service are inventory control systems and time shared software systems.
A third type of data service involves bulk data transactions which are either periodically polled from a computer center or periodically transmitted to a computer when a buffer is full. Such a system is typically not interactive since data entry and data responses do not occur in real time but are delayed until the buffer is full or until the unit is polled. Polling arrangements can be inefficient because the telephone line is periodically tied up on each polling inquiry to the customer whether or not the customer has any data to transmit. Such a polling arrangement also makes it difficult to share voice and data over a common facility.
The prior art single data transaction or bulk data services require sending dialing and user identification protocol information with each data transfer. If the number of data transfers per unit time is significant a multiple interactive type data service is often utilized enabling several data transfers during one data call. Only one protocol is required, since only one data call is established to enable the multiple data transfers. While the multiple interactive type data service reduces the protocol overhead required for data transfers, it does so at the expense of tying up the customer's loop during the period of time when no data is being transferred. In some arrangements, such as inventory control systems, the data call can last for an hour or more. The resulting long holding times (the time a line is off-hook) for each data call creates a call blocking problem for the telephone network.
The data call holding time problem is not as serious for the single data transaction or bulk data services. However, since the single data transaction data service requires the establishment of a telephone connection (data call) for each data transfer the number of calls which can be made in a given time period is limited. Hence, such a service is not recommended for high usage data services. The bulk data services accumulate data for a period of time and send it during one data call. Because of the delay involved, this service cannot effectively operate as an interactive real-time data service. Thus, for customers requiring multiple interactive real-time data transfer capability the only data service available in the prior art utilizes long holding time data calls which create significant call blocking problems for the telephone network.
The resulting telephone network blocking problems occur because line concentrators of the telephone switching equipment are designed to match the short holding time traffic characteristics of the telephone customer's voice communications. These line concentrators are designed to efficiently concentrate many customer lines to a fewer number of expensive trunk circuits which require high usage for economic telephone service. When this average holding time increases drastically, due to increased numbers of data calls each having long holding times, the traffic density increases and the probability of one party being unable to reach another party (i.e., blocking probability) increases.
The blocking problem is more acute at the telephone termination switching office of the computer vendor who operates the data communication service. This vendor is usually equipped with many central office lines to handle his own traffic requirements. When these lines are blocked by data calls having long holding times, the resulting congestion at the terminating switching office can back up into other telephone offices creating additional telephone network congestion. Typically, this blocking problem is not solved by using separate data communication networks since these data networks generally utilize the local telephone facilities rather than private facilities for user data distribution. The final result of this increased blocking probability is a degradation of both voice and data services over the telephone network as well as a degradation in the services offered by the computer vendor.
An additional effect of the call blocking problem is that the data communication user has no voice telephone service while he is currently making a data call. The situation is particularly troublesome for the small business customer who relies on his voice telephone to do business. Thus, most customers who require frequent data communications install a separate data communication line in addition to their voice telephone line.
Thus, it is a general problem to decrease the telephone service degradation resulting from allowing data services access to the telephone network.